1. Field of the Invention
The present invention relates to apparatus and methods for suppressing vortex-induced vibrations (VIV). In another aspect, the present invention relates to apparatus and methods for suppressing VIV while also suppressing the growth of aquatic organisms on the VIV suppressing apparatus. In even another aspect, the present invention relates to apparatus for suppressing VIV comprised of copper for suppressing the growth of aquatic organisms on the surface of the apparatus.
2. Description of the Related Art
Whenever a bluff body, such as a cylinder, experiences a current in a fluid, it is possible for the body to experience vortex-induced vibrations (VIV). These vibrations are caused by oscillating hydrodynamic forces on the surface which can cause substantial vibrations of the structure, especially if the forcing frequency is at or near a structural natural frequency. The vibrations are largest in the transverse (to flow) direction; however, in-line vibrations can also cause stresses which are sometimes larger than those in the transverse direction.
Drilling for and/or producing hydrocarbons or the like from subterranean deposits which exist under a body of water exposes underwater drilling and production equipment to water currents and the possibility of VIV. Equipment exposed to VIV includes structures ranging from the smaller tubes of a riser system, anchoring tendons, or lateral pipelines to the larger underwater cylinders of the hull of a minispar or spar floating production system (hereinafter xe2x80x9csparxe2x80x9d).
Risers are discussed here as a non-exclusive example of an aquatic element subject to VIV. A riser system is used for establishing fluid communication between the surface and the bottom of a water body. The principal purpose of the riser is to provide a fluid flow path between a drilling vessel and a well bore and to guide a drill string to the well bore.
A typical riser system normally consists of one or more fluid-conducting conduits which extend from the surface to a structure (e.g., wellhead) on the bottom of a water body. For example, in the drilling of a submerged well, a drilling riser usually consists of a main conduit through which the drill string is lowered and through which the drilling mud is circulated from the lower end of the drill string back to the surface. In addition to the main conduit, it is conventional to provide auxiliary conduits, e.g., choke and kill lines, etc., which extend parallel to and are carried by the main conduit.
Also, the newly developed spar production facilities are used in aquatic environments of great depths. Aquatic environments is used here to describe water environments of any salinity. Strong water currents often occur at these greater depths in ocean environments. The hulls of spar production facilities, therefore, can be exposed to excessive vortex-induced vibrations.
Methods to reduce vibrations caused by vortex shedding from aquatic structures can operate by modifying the boundary layer of the flow around the structure to prevent the correlation of vortex shedding along the length of the structure. Examples of such methods include the inclusion of helical strakes around a structure, or axial rod shrouds and perforated shrouds.
The use of strakes and shrouds in aquatic environments exposes them to being colonized by aquatic animals and plants that anchor themselves to these structures. Mature growth of these organisms on strakes and shrouds changes the contours of these VIV suppression devices and can diminish their effectiveness in VIV suppression, and perhaps in some instances result in greater VIV. The location of these VIV suppression devices deep in aquatic environments makes it difficult to remove aquatic organisms. Aquatic organisms can be removed from VIV suppression devices by divers working from water craft, defined herein as any vehicle that can travel on or in water. After removal, the aquatic organisms immediately begin to re-colonize the structures.
However, in spite of the above advancements, there still exists a need in the art for improved apparatus and methods for VIV suppression.
There is another need in the art for apparatus and methods for strakes and other VIV suppression devices which resist the growth of aquatic organisms on their surfaces.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
It is an object of the present invention to provide for improved apparatus and methods for VIV suppression.
It is another object of the present invention to provide for apparatus and methods for strakes and other VIV suppression devices which resist the growth of aquatic organisms on their surfaces.
These and other objects of the present invention will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.
According to one embodiment of the present invention, there is provided a system for suppressing VIV. The system generally includes a first flange, having a first set of two or more members, and a second flange having a second set of two or more members. The system further includes an elongated strake member with the elongated strake member connected to the first flange and the second flange. At least a portion of the system comprises copper in the range of about 1 wt % to about 100 wt %, based on the total weight of that portion. In a more specific embodiment of this embodiment, the system may also include an elongated sleeve positioned within the first and second flanges and a marine element positioned within the elongated sleeve.
According to another embodiment of the present invention, there is provided a method of modifying a structure intended to operate while at least partially immersed in an aquatic environment. The method generally includes arranging a plurality of elongated members around at least a portion of the structure, wherein at least a portion of the elongated members comprise copper in the range of about 1 wt % to about 100 wt % copper.